The discovery of four viruses to affect grapes and one to strike wisteria alarmed pest and disease experts at the Royal Horticultural Society.

Two of the most damaging viruses to affect grape vines have been transmitted by mealybugs and scale insects.

As well as threatening grapes grown by amateur gardeners, new viruses have the potential to devastate the UK’s grapevine industry.

The threat came to light when the RHS released its list of the top 10 plant diseases reported during 2014.

Honey fungus was the most commonly diagnosed garden disease for the 19th year running (see panel, below).

It affects a wide variety of plants, particularly woody and herbaceous perennials.

Described as a “silent killer,” it can infect host plants long before symptoms appear.

And even after infected plants have been removed, residues of the fungus left in the soil can infect new plants.

Senior plant pathologist at the RHS, Matthew Cromey, said: “We are investigating how to accelerate the breakdown of honey fungus residues, to allow gardeners to replant susceptible plants more quickly in areas that have been affected.”

Warm temperatures and high rainfall in 2014 created ideal conditions for diseases that attack leaves to thrive.

Enquiries about foliar diseases, including rusts and leaf spots, “increased significantly” in 2014, said the RHS, pushing rust diseases up four places in the chart from 7th in 2013 to 4th last year.

Box blight caused much concern last year, too. The blight, which turns box bald and brown, maintained its position in second place in the RHS disease chart.